4

The environment often contains as astonishingassortment of organisms that interact with each otherand are interdependent in a variety of ways. Consider fora moment a salt marsh in the Chesapeake Bay on theEast Coast of the United States. This bay is one of theworld’s richest estuaries, semi-enclosed bodies of waterfound where fresh water from a river drains into theocean. Estuaries are under the influence of tides andgradually change from unsalty fresh water to salty oceanwater. In the Chesapeake Bay this change results inthree distinct marsh communities: freshwater marshesat the head of the bay, brackish (moderately salty)marshes in the middle bay region, and salt marshes onthe ocean side of the bay. Each community has its owncharacteristic organisms.

A Chesapeake Bay salt marsh consists of floodedmeadows of cordgrass (Spartina; see photo). Few otherplants are found because high salinity and twice-dailytidal inundations produce a challenging environment towhich only a few plants have adapted. Nutrients such asnitrates and phosphates, much of them from treatedsewage and agriculture, drain into the marsh from theland and promote rapid growth of both cordgrass andmicroscopic algae suspended in the water. These organ-isms are eaten directly by some animals, and when theydie, their remains provide food for other salt marshinhabitants.

A casual visitor to a salt marsh would observe twodifferent types of animal life: insects and birds. Insects,

Cordgrass in a Chesapeake Bay salt

marsh.

63

Learning Objectives

After you have studied this chapter you should be able to:

1. Define ecology and distinguish among the following

ecological levels: population, community, ecosystem, land-

scape, and biosphere.

2. Define energy, and explain how it is related to work and

to heat.

3. Use examples to contrast potential energy and kinetic

energy.

4. State the first and second laws of thermodynamics, and

discuss the implications of these laws as they relate to

organisms.

5. Write summary reactions for photosynthesis and respi-

ration, and contrast these two biological processes.

6. Describe the communities around hydrothermal vents

and explain the source of energy that sustains them.

7. Summarize how energy flows through a food web, using

the terms producer, consumer, and decomposer.

8. Explain some of the impacts humans have had on the

Antarctic food web.

9. Draw and explain typical pyramids of numbers, bio-

mass, and energy.

10. Distinguish between gross primary productivity and

net primary productivity, and discuss human impact on the

latter.

Ecosystems and Energy

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particularly mosquitoes and horseflies, number in the mil-lions. Birds nesting in the salt marsh include seasidesparrows, laughing gulls, and clapper rails. Study the saltmarsh carefully and you will find it has numerous otherspecies. Large numbers of invertebrates, such asshrimps, lobsters, crabs, barnacles, worms, clams, andsnails, seek refuge in the water surrounding the cord-grass. Here they eat, hide from predators to avoid beingeaten, and reproduce. Many invertebrates gather on theshore between the high-tide mark and the low-tide markbecause food—detritus, algae, protozoa, and worms—isabundant there.

Almost no amphibians inhabit salt marshes (the saltywater dries out their skin), but a few reptiles, such as thenorthern diamondback terrapin (a semi-aquatic turtle),have adapted. It spends its time basking in the sun orswimming in the water searching for food—snails, crabs,worms, insects, and fish. Although a variety of snakesabound in the dry areas adjacent to salt marshes, only thenorthern water snake, which preys on fish, is adapted tosalty water.

The meadow vole is a small rodent that lives in thesalt marsh. It constructs a nest of cordgrass on the groundabove the high-tide zone. Meadow voles are excellentswimmers and scamper about the salt marsh day andnight. Their diet consists mainly of insects and the leaves,stems, and roots of cordgrass.

The Chesapeake Bay marshes are an important nurs-ery for numerous marine fishes—spotted sea trout,Atlantic croaker, striped bass, and bluefish, to name just afew. These fishes typically spawn (reproduce) in the openocean, and the young then enter the estuary, where theyeat smaller fish and invertebrates and grow into juveniles.

Add to all these visible plant and animal organismsthe unseen microscopic world of the salt marsh, whichincludes countless numbers of protozoa, fungi, and bac-teria, and you can begin to appreciate the complexity of asaltmarsh community.

WHAT IS ECOLOGY?

The concept of ecology was first developed in the 19thcentury by Ernst Haeckel, who also devised its name—ecofrom the Greek word for “house” and logy from theGreek word for “study.” Thus, ecology literally means “thestudy of one’s house.” The environment—one’s house—consists of two parts, the biotic (living) environment,which includes all organisms, and the abiotic (nonliving,or physical) environment, which includes such physicalfactors as living space, temperature, sunlight, soil, wind,and precipitation. Ecology, then, is the study of theinteractions among organisms and between organismsand their abiotic environment.

The focus of ecology can be local or global, specificor generalized, depending on what questions the scientistis asking and trying to answer. One ecologist mightdetermine the temperature or light requirements of a sin-gle species of oak, another might study all the organismsthat live in a forest where the oak is found, and anothermight examine how nutrients flow between the forest andsurrounding communities.

Ecology is the broadest field within the biologicalsciences, and it is linked to every other biological disci-pline. The universality of ecology also links subjects thatare not traditionally part of biology. Geology and earthscience are extremely important to ecology, especiallywhen ecologists examine the physical environment ofplanet Earth. Chemistry and physics are also important.Because humans are biological organisms, all of ouractivities have a bearing on ecology. Even economics andpolitics have profound ecological implications, as waspresented in Chapter 3.

How does the field of ecology fit into the organiza-tion of the biological world? As you may know, one of thecharacteristics of life is its high degree of organization(Figure 4.1). Atoms are organized into molecules, whichare organized into cells. In multicellular organisms, cellsare organized into tissues, tissues into organs such as abone or stomach, organs into body systems such as thenervous system and digestive system, and body systemsinto individual organisms such as dogs, ferns, and so on.

Ecologists are most interested in the levels of biolog-ical organization that include or are above the level of theindividual organism (see “Mini-Glossary: EcologyTerms”). Organisms occur in populations, which aremembers of the same species that live together in thesame area at the same time. (A species is a group of sim-ilar organisms whose members freely interbreed with oneanother in the wild to produce fertile offspring; membersof one species do not interbreed with other species oforganisms.) A population ecologist might study a popula-

64 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

Ecology Terms

population: A group of organisms of the same species that livetogether in the same area at the same time.

community: All the populations of different species that are livingtogether in the same area at the same time.

ecosystem: A community and its physical environment. Includes allthe interactions among organisms and between organisms and theirabiotic environment. These interactions form a complex network ofenergy flow and materials cycling.

landscape: A region that includes several ecosystems.

biosphere: The layer of Earth containing all living organisms. As anecological system, the biosphere interacts with the land, the water,and the atmosphere.

M I N I - G L O S S A R Y

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W H AT I S E C O LO G Y ? 6 5

Atoms

Molecule

OxygenHydrogen

Water

Cells

Biosphere

Bodysystem

Organism

Cell

Tissue

Organ

Landscape

Community andEcosystem

Population

Figure 4.1 Levels of biological organization. Starting at the simplest level, atoms are organ-ized into molecules, which are organized into cells. Cells are organized into tissues, tissues intoorgans, organs into body systems, and body systems into individual multicellular organisms. Agroup of individuals of the same species is a population. Populations of different species interactto form communities. A community and its abiotic environment are an ecosystem, whereas aregion with several ecosystems is a landscape. The layer of Earth containing all living organismscomprises the biosphere. Ecologists study the highest levels of biological organization: individ-ual organisms, populations, communities, ecosystems, landscapes, and the biosphere.

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tion of polar bears or a population of marsh grass. Popu-lations, particularly human populations, are so importantin environmental science that we devote two chapters,Chapters 8 and 9, to their study.

Populations are organized into communities. Acommunity is a natural association that consists of all thepopulations of different species that live and interacttogether within an area at the same time. Different com-munities are characterized by the number and kinds ofspecies that live there, along with their relationships withone another. A community ecologist might study howorganisms interact with one another—including feedingrelationships (who eats whom)—in an alpine meadowcommunity or in a coral reef community (Figure 4.2).

Ecosystem is a more inclusive term than communitybecause an ecosystem is a community together with itsphysical environment. Thus, an ecosystem includes notonly all the biotic interactions of a community but alsothe interactions between organisms and their abioticenvironment. In an ecosystem, all of the biological,physical, and chemical components of an area form anextremely complicated interacting network of energyflow and materials cycling. An ecosystem ecologistmight examine how energy, nutrients, organic (carbon-containing) materials, and water affect the organismsliving in a desert community or a coastal bay ecosystem.

The ultimate goal of ecosystem ecologists is tounderstand how ecosystems function. This is not a sim-ple task, but it is important because ecosystem processescollectively regulate global cycles of water, carbon, nitro-gen, and oxygen that are essential to the survival ofhumans and all other organisms (see Chapter 6). Ashumans increasingly alter ecosystems for their own uses,

the natural functioning of ecosystems is changed, and weneed to know if these changes will affect the sustainabil-ity of our life-support system.

Landscape ecology is a relatively new subdisciplinein ecology that studies the connections among ecosys-tems found in a particular region. Consider a landscapeconsisting of a forest ecosystem located adjacent to apond ecosystem. One connection between these twoecosystems might be great blue herons, which eat fish,frogs, insects, crustaceans, and snakes along the shallowwater of the pond but often build nests and raise theiryoung in the secluded treetops of the nearby forest.Landscapes, then, are based on larger land areas thatinclude several ecosystems.

The layer of Earth that contains all living organisms isknown as the biosphere. The organisms of the bios-phere—Earth’s communities, ecosystems, and land-scapes—depend on one another and on the other realms ofthe Earth’s physical environment: the atmosphere, hydros-phere, and lithosphere (Figure 4.3). The atmosphere isthe gaseous envelope surrounding the Earth; the hydros-phere is the Earth’s supply of water—liquid and frozen,fresh and salty; and the lithosphere is the soil and rock ofEarth’s crust. Ecologists who study the biosphere examinethe complex global interrelationships among the Earth’satmosphere, land, water, and organisms.

The biosphere teems with life. Where do these organ-isms get the energy to live? And how do they harness thisenergy? We now examine the importance of energy toorganisms, which can continue to survive only as long asthe environment continuously supplies them with energy.We will revisit the importance of energy as it relates tohuman endeavors in many chapters throughout this text.

66 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

Figure 4.2 Coral reef commu-

nity. Coral reef communitieshave the greatest number ofspecies and are the most complexaquatic community. A coral reefcommunity in the Indian Oceanoff the coast of Maldives is shown.

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THE ENERGY OF LIFE

Energy is the capacity or ability to do work. In organ-isms, the biological work that requires energy includesprocesses such as growing, moving, reproducing, andmaintaining and repairing damaged tissues. Energy existsin several different forms: chemical, radiant, heat,mechanical, nuclear, and electrical (see “Mini-Glossary:Forms of Energy”). Biologists generally express energy inunits of work (kilojoules, kJ) or units of heat energy(kilocalories, kcal). One kilocalorie, which is the energyrequired to raise the temperature of 1 kg of water by 1°C,equals 4.184 kJ.

Energy can exist as stored energy—called potentialenergy—or as kinetic energy, the energy of motion(Figure 4.4). You can think of potential energy as an

T H E E N E R G Y O F L I F E 6 7

Figure 4.3 The northern tip of

Palawan Island, Philippines,

shows Earth’s four realms. Theatmosphere contains cumulusclouds, which indicate warm,moist air. The jagged spires ofrock, formed from volcanic lavaflows that have eroded over time,represent the lithosphere. Theshallow water of the bay repre-sents the hydrosphere. The bios-phere includes the greenvegetation and human settlementalong the shore as well as the coralreefs visible as darker areas in thebay.

Forms of Energy

chemical energy: Energy stored in the chemical bonds of molecules.For example, food contains chemical energy.

radiant, or solar, energy: Energy transported from the sun as elec-tromagnetic waves.

heat energy: Thermal energy that flows from an object with a highertemperature (the heat source) to an object with a lower temperature(the heat sink).

mechanical energy: Energy in the movement of matter.

nuclear energy: Energy found within atomic nuclei (discussed inChapter 11).

electrical energy: Energy that flows as charged particles.

M I N I - G L O S S A R Y

POTENTIAL

KINETIC

Figure 4.4 Potential and kinetic energy. Potential energy isstored in the drawn bow and is converted to kinetic energy asthe arrow speeds toward its target.

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arrow on a drawn bow. When the string is released, thispotential energy is converted to kinetic energy as themotion of the bow propels the arrow. Similarly, the cord-grass that a meadow vole eats has chemical potentialenergy, some of which is converted to kinetic energy andheat as the meadow vole swims in the salt marsh. Thus,energy can change from one form to another.

The study of energy and its transformations is calledthermodynamics. When considering thermodynamics,scientists use the term system to refer to an object that isbeing studied. The rest of the universe other than thesystem being studied is known as the surroundings. Aclosed system is one that does not exchange energy withits surroundings, whereas an open system is one that canexchange energy with its surroundings (Figure 4.5).

Regardless of whether a system is open or closed,there are two laws about energy that apply to all things inthe universe: the first and second laws of thermodynamics.

The First Law of Thermodynamics

According to the first law of thermodynamics, energycannot be created or destroyed, although it can be trans-formed from one form to another. As far as we know, theenergy present in the universe at its formation, approxi-mately 15 billion to 20 billion years ago, equals theamount of energy present in the universe today. This isall the energy that can ever be present in the universe.Similarly, the energy of any system and its surroundingsis constant. A system may absorb energy from its sur-roundings, or it may give up some energy into its sur-roundings, but the total energy content of that systemand its surroundings is always the same.

As specified by the first law of thermodynamics, then,an organism cannot create the energy that it requires tolive. Instead, it must capture energy from the environmentto use for biological work, a process involving the transfor-mation of energy from one form to another. In photosyn-thesis, plants absorb the radiant energy of the sun andconvert it into the chemical energy contained in the bondsof carbohydrate (sugar) molecules. Similarly, some of thatchemical energy may later be transformed by an animalthat eats the plant into the mechanical energy of musclecontraction, enabling it to walk, run, slither, fly, or swim.

The Second Law of Thermodynamics

As each energy transformation occurs, some of theenergy is changed to heat energy that is then releasedinto the cooler surroundings. This energy can neveragain be used by any organism for biological work; it is“lost” from the biological point of view. However, it isnot really gone from a thermodynamic point of viewbecause it still exists in the surrounding physical environ-ment. The use of food to enable us to walk or run doesnot destroy the chemical energy that was once present inthe food molecules. After we have performed the task ofwalking or running, the energy still exists in the sur-roundings as heat energy.

The second law of thermodynamics can be statedmost simply as follows: Whenever energy is convertedfrom one form to another, some usable energy—that is,energy available to do work—is degraded into heat, aless-usable form that disperses into the environment. As aresult, the amount of usable energy available to do workin the universe decreases over time.

It is important to understand that the second law ofthermodynamics is consistent with the first law; that is,the total amount of energy in the universe is not decreas-ing with time. However, the total amount of energy inthe universe that is available to do biological work isdecreasing over time.

Less-usable energy is more diffuse, or disorganized.Entropy is a measure of this disorder or randomness;organized, usable energy has low entropy, whereas disor-ganized energy such as heat has high entropy. Entropy iscontinuously increasing in the universe in all naturalprocesses. It may be that at some time, billions of yearsfrom now, all energy will exist as heat uniformly distrib-uted throughout the universe. If that happens, the universeas a closed system will cease to operate because no workwill be possible. Everything will be at the same tempera-ture, so there will be no way to convert the thermal energyof the universe into usable mechanical energy.

Another way to explain the second law of thermody-namics, then, is that entropy, or disorder, in a systemspontaneously tends to increase over time. (The wordspontaneously in this context means that entropy occursnaturally rather than being caused by some externalinfluence.)

68 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

(a) Closed system

Sun

(b) Open system

Figure 4.5 Closed and open systems, with regard to energy.

(a) Energy is not exchanged between a closed system and itssurroundings. (b) Energy is exchanged between an open sys-tem and its surroundings. Earth is an open system because itreceives energy from the sun.

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As a result of the second law of thermodynamics, noprocess requiring an energy conversion is ever 100% effi-cient, because much of the energy is dispersed as heat,resulting in an increase in entropy. For example, an auto-mobile engine, which converts the chemical energy ofgasoline to mechanical energy, is between 20% and 30%efficient. That is, only 20% to 30% of the original energystored in the chemical bonds of the gasoline molecules isactually transformed into mechanical energy, or work. Inour cells, energy utilization for metabolism is about 50%efficient; the remaining energy is released to the sur-roundings as heat.

Organisms have a high degree of organization, and atfirst glance, they appear to refute the second law of ther-modynamics. As organisms grow and develop, they main-tain a high level of order and do not appear to becomemore disorganized. However, organisms are able tomaintain their degree of order over time only with theconstant input of energy. That is why plants must photo-synthesize and animals must eat food.

Photosynthesis and Cellular Respiration

Photosynthesis is the biological process in which lightenergy from the sun is captured and transformed into thechemical energy of carbohydrate (sugar) molecules. Pho-tosynthetic pigments such as chlorophyll, which is greenand gives plants their green color, absorb radiant energy.This energy is used to manufacture a carbohydrate calledglucose (C6H12O6) from carbon dioxide (CO2) and water(H2O), with the liberation of oxygen (O2):

6CO2 + 12 H2O + radiant energy → C6H12O6 + 6H2O + 6O2

The chemical equation for photosynthesis is read as fol-lows: 6 molecules of carbon dioxide plus 12 molecules ofwater plus light energy are used to produce 1 molecule ofglucose plus 6 molecules of water plus 6 molecules ofoxygen. (See Appendix I for a review of basic chemistry.)

Plants, some bacteria, and algae (photosyntheticaquatic organisms that range from single cells to sea-weeds well over 50 m in length) perform photosynthesis,a process that is essential for almost all life. Photosynthe-sis provides these organisms with a ready supply ofenergy in carbohydrate molecules that they can use as theneed arises. The energy can also be transferred from oneorganism to another—for instance, from plants to theorganisms that eat plants (Figure 4.6). Oxygen, which isrequired by many organisms when they break down glu-cose or similar foods by cellular respiration, is a by-prod-uct of photosynthesis.

The chemical energy that plants store in carbohy-drates and other molecules is released within cells ofplants, animals, or other organisms through cellularrespiration. In this process, molecules such as glucoseare broken down in the presence of oxygen and water

into carbon dioxide and water, with the release ofenergy:

C6H12O6 + 6O2 + 6H2O → 6CO2 + 12H2O + energy

Cellular respiration makes the chemical energy stored inglucose and other food molecules available to the cell forbiological work, such as moving around, courting, andgrowing new cells and tissues. All organisms, includinggreen plants, respire to obtain energy. Some organisms,however, do not use oxygen for this process. Some typesof bacteria that live in waterlogged soil, stagnant ponds,or animal intestines respire in the absence of oxygen.

Life Without the Sun

The sun is the energy source for almost all of the bios-phere. A notable exception was discovered in 1977, whenan oceanographic expedition aboard the submersibleresearch craft Alvin studied the Galapagos Rift, a deepcleft in the ocean floor off the coast of Ecuador. Theexpedition revealed, on the floor of the deep ocean, aseries of hydrothermal vents where seawater apparentlyhad penetrated and been heated by the hot rocks below.During its time within the Earth, the water had been

CASE·IN·POINT

T H E E N E R G Y O F L I F E 6 9

Figure 4.6 Black-tailed prairie dog (Cynomys ludovicianus).

The chemical energy produced by photosynthesis and storedin seeds and leaves is transferred to the black-tailed prairie dogas it eats.

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charged with inorganic compounds, including hydrogensulfide (H2S).

At the tremendous depth (greater than 2,500 m, or8,202 ft) of the Galapagos Rift, there is no light for pho-tosynthesis. But the hot springs support a rich andbizarre ecosystem that contrasts with the surrounding“desert” of the deep-ocean floor. Most of the speciesfound in these oases of life were new to science. Giant,bloodred tube worms almost 3 m (10 ft) in length clusterin great numbers around the vents (Figure 4.7). Otheranimals around the hydrothermal vents include clams,crabs, barnacles, and mussels.

Scientists initially wondered what the ultimatesource of energy for the species in this dark environmentis. Most deep-sea ecosystems depend on the organicmaterial that drifts down from surface waters; that is,they depend on energy derived from photosynthesis. Butthe Galapagos Rift ecosystem and other hydrothermalvent ecosystems are too densely clustered and too pro-ductive to be dependent on chance encounters withorganic material from surface waters.

The base of the food web in these aquatic oases con-sists of certain bacteria that can survive and multiply inwater so hot (exceeding 200°C, or 392°F) that it wouldnot even remain in liquid form were it not under suchextreme pressure. These bacteria function as producers,but they do not photosynthesize. Instead, they chemo-synthesize, obtaining energy and making carbohydratemolecules from inorganic raw materials. Chemosyntheticbacteria possess enzymes (organic catalysts) that causethe inorganic molecule hydrogen sulfide to react withoxygen, producing water and sulfur or sulfate. Suchchemical reactions provide the energy required to sup-port these bacteria and other organisms in deep-oceanhydrothermal vents. Many of the Galapagos Rift animalsconsume the bacteria directly by filter feeding. Others,such as the giant tube worms, are supplied energy fromchemosynthetic bacteria that live symbiotically insidetheir bodies.

THE FLOW OF ENERGY THROUGH

ECOSYSTEMS

We have seen that, with the exception of hydrothermalvent ecosystems, energy enters ecosystems as radiantenergy (sunlight), some of which is trapped by plantsduring photosynthesis. The energy, now in chemicalform, is stored in the bonds of organic molecules such asglucose. Animals obtain their energy by eating plants orby eating animals that ate plants. All organisms—plants,animals, and microorganisms—respire to obtain some ofthe energy in organic molecules. When these moleculesare broken apart by cellular respiration, the energybecomes available to do work such as repairing tissues,producing body heat, or reproducing. As the work isaccomplished, the energy escapes the organism and dissi-pates into the environment as heat (recall the second lawof thermodynamics). Ultimately, this heat energy radiatesinto space. Thus, once energy has been used by an organ-ism, it becomes unusable. The movement of energy justdescribed—in a one-way direction through an ecosys-tem—is known as energy flow.

Producers, Consumers, and Decomposers

The organisms of an ecosystem can be divided into threecategories on the basis of how they obtain nourishment:producers, consumers, and decomposers (Figure 4.8).Virtually all ecosystems contain representatives of allthree groups, which interact extensively, both directlyand indirectly, with one another.

Producers, also called autotrophs (Greek auto,“self,” and tropho, “nourishment”), manufacture complexorganic molecules from simple inorganic substances,generally carbon dioxide and water, usually using theenergy of sunlight to do so. In other words, most produc-

70 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

Figure 4.7 A hydrothermal vent ecosystem. Bacteria living inthe tissues of these tube worms extract energy from hydrogensulfide to manufacture organic compounds. Because theseworms lack digestive systems, they depend on the organiccompounds provided by the bacteria, along with materials fil-tered from the surrounding water. Also visible in the photo-graph are some filter-feeding clams (yellow) and a crab (white).

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ers perform the process of photosynthesis. By incorpo-rating the chemicals they manufacture into their ownbodies, producers become potential food resources forother organisms. Whereas plants are the most significantproducers on land, algae and certain types of bacteria areimportant producers in aquatic environments. In the salt-marsh ecosystem discussed in the chapter introduction,cordgrass, algae, and photosynthetic bacteria are theimportant producers.

Animals are consumers; that is, they use the bodies ofother organisms as a source of food energy and bodybuild-ing materials. Consumers are also called heterotrophs(Greek heter, “different,” and tropho, “nourishment”).Consumers that eat producers are called primary con-sumers, which usually means that they are exclusivelyherbivores (plant eaters). Rabbits and deer are examplesof primary consumers, as is the marsh periwinkle, a type ofsnail that feeds on algae in the saltmarsh ecosystem.

Secondary consumers eat primary consumers,whereas tertiary consumers eat secondary consumers.Both secondary and tertiary consumers are flesh-eatingcarnivores that eat other animals. Lions and spiders areexamples of carnivores, as are the northern diamondbackterrapin and the northern water snake in the saltmarshecosystem. Other consumers, called omnivores, eat a

variety of organisms, both plant and animal. Bears, pigs,and humans are examples of omnivores; the meadowvole, which eats both insects and cordgrass in the salt-marsh ecosystem, is also an omnivore.

Some consumers, called detritus feeders or detriti-vores, consume detritus, which is organic matter thatincludes animal carcasses, leaf litter, and feces. Detritusfeeders, such as snails, crabs, clams, and worms, are espe-cially abundant in aquatic environments, where they bur-row in the bottom muck and consume the organic matterthat collects there. Marsh crabs are detritus feeders in thesaltmarsh ecosystem. Earthworms are terrestrial (land-dwelling) detritus feeders, as are termites, beetles, snails,and millipedes. Detritus feeders work together withmicrobial decomposers to destroy dead organisms andwaste products. An earthworm actually eats its waythrough the soil, digesting much of the organic mattercontained there.

Decomposers, also called saprotrophs (Greeksapro, “rotten,” and tropho, “nourishment”), are microbialheterotrophs that break down dead organic material anduse the decomposition products to supply themselveswith energy. They typically release simple inorganic mol-ecules, such as carbon dioxide and mineral salts, that canthen be reused by producers. Bacteria and fungi are

T H E F LO W O F E N E R G Y T H R O U G H E C O S Y S T E M S 7 1

Energy

CONSUMER

Food

CONSUMER

Plant litter, wastes anddead bodies

DECOMPOSERS

PRODUCER

Energydispersedas heat

Energydispersedas heat

Energydispersedas heat

Energydispersedas heat

Figure 4.8 Producers, con-

sumers, and decomposers.

During photosynthesis, produc-ers use the energy from sunlightto make complex moleculesfrom carbon dioxide and water.Consumers obtain energy whenthey eat producers or consumersthat ate producers. Wastes anddead organic material supplydecomposers with energy. Dur-ing every energy transaction,some energy is lost to biologicalsystems as it is dispersed as heat.

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important examples of decomposers. Sugar-metabolizingfungi first invade dead wood and consume the wood’ssimple carbohydrates, such as glucose and maltose.When these carbohydrates are exhausted, other fungi,often aided by termites with symbiotic bacteria in theirguts, complete the digestion of the wood by breakingdown cellulose, a complex carbohydrate that is the maincomponent of wood.

Ecosystems such as the Chesapeake Bay salt marshcontain a balanced representation of all three ecologicalcategories of organisms—producers, consumers, anddecomposers—and all of these have indispensable rolesin ecosystems. Producers provide both food and oxygenfor the rest of the community. Consumers play an impor-tant role by maintaining a balance between producersand decomposers. Detritus feeders and decomposers arenecessary for the long-term survival of any ecosystembecause, without them, dead organisms and waste prod-ucts would accumulate indefinitely. Without microbialdecomposers, important elements such as potassium,nitrogen, and phosphorus would remain permanently indead organisms and therefore be unavailable for use bynew generations of organisms.

The Path of Energy Flow: Who Eats Whom in Ecosystems

In an ecosystem, energy flow occurs in food chains, inwhich energy from food passes from one organism to the

next in a sequence (Figure 4.9). Each level, or “link,” in afood chain is called a trophic level (recall that the Greektropho means nourishment). The first trophic level isformed by producers (organisms that photosynthesize),the second level by primary consumers (herbivores), thethird level by secondary consumers (carnivores), and soon. At every step in a food chain are decomposers, whichrespire organic molecules in the carcasses and bodywastes of all members of the food chain.

Simple food chains rarely occur in nature, becausefew organisms eat just one kind of organism. More typ-ically, the flow of energy and materials through anecosystem takes place in accordance with a range ofchoices of food for each organism involved. In anecosystem of average complexity, numerous alternativepathways are possible. A hawk eating a rabbit is a differ-ent energy pathway than a hawk eating a snake. Thus, afood web, which is a complex of interconnected foodchains in an ecosystem, is a more realistic model of theflow of energy and materials through ecosystems (Fig-ure 4.10).

The most important thing to remember aboutenergy flow in ecosystems is that it is linear, or one-way. That is, energy can move along a food chain orfood web from one organism to the next as long as ithas not been used to do biological work. Once energyhas been used by an organism, however, it is lost as heatand is unavailable for use by any other organism in theecosystem.

72 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

Energyfromsun

Heat Heat Heat Heat Heat

Firsttrophic level:Producers

Secondtrophic level:

Primaryconsumers

Thirdtrophic level:Secondaryconsumers

Fourthtrophic level:

Tertiaryconsumers Decomposers

Figure 4.9 Energy flow through a food chain. Energy enters ecosystems from an externalsource (the sun), flows linearly—in a one-way direction—through ecosystems and exits as heatloss. Much of the energy acquired by a given level of the food chain is used for respiration atthat level and escapes into the surrounding environment as heat. This energy, as stipulated bythe second law of thermodynamics, is unavailable to the next level of the food chain.

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How Humans Have Affected theAntarctic Food Web

Although the icy waters around Antarctica may seem tobe an inhospitable environment, a complex food web isfound there. The base of the food web is microscopicalgae, which are present in vast numbers in the well-litnutrient-rich water. These marine algae are eaten by ahuge population of herbivores—tiny shrimplike animalscalled krill (Figure 4.11). Krill in turn support a varietyof larger animals. One of the main consumers of krill isbaleen whales, which filter krill out of the frigid water.Baleen whales include blue whales, humpback whales,and right whales. Krill are also consumed in great quan-tities by squid and fish. These in turn are eaten by othercarnivores: toothed whales such as the sperm whale; ele-phant seals and leopard seals; king penguins andemperor penguins; and birds such as the albatross andpetrel.

Humans have had an impact on the complex Antarc-tic food web, as they have on most other ecosystems.Before the advent of whaling, baleen whales consumedhuge quantities of krill. During the past 150 years—untila 1986 global ban on hunting all large whales—whalingsteadily reduced the number of large baleen whales inAntarctic waters. Some whale populations are so deci-mated that they are on the brink of extinction. As a resultof fewer whales eating krill, more krill have been avail-able for other krill-eating animals, whose populationshave increased. Seals, penguins, and smaller baleenwhales have replaced the large baleen whales as the maineaters of krill.

Now that commercial whaling is regulated, it ishoped that the number of large baleen whales will slowlyincrease, and that appears to be the case for many species.As of the late 1990s, the only whale population that didnot appear to be growing in response to the moratoriumon whaling was the southern blue whale. It is not knownwhether baleen whales will return to or be excluded fromtheir former position of dominance in terms of krill con-sumption in the food web. Biologists will monitorchanges in the Antarctic food web as the whale popula-tions recover.

Recently, a human-related change has developed inthe atmosphere over Antarctica that has the potential tocause far greater effects on the entire Antarctic foodweb—thinning of the ozone layer in the stratosphericregion of the atmosphere. This ozone thinning allowsmore of the sun’s ultraviolet radiation to penetrate to theEarth’s surface. Ultraviolet radiation contains moreenergy than visible light. It is so energetic that it canbreak the chemical bonds of some biologically importantmolecules, such as deoxyribonucleic acid (DNA).

Scientists are concerned that ozone thinning overAntarctica may damage the algae that form the base ofthe food web in the southern ocean. A 1992 study con-firmed that increased ultraviolet radiation is penetratingthe surface waters around Antarctica and that algal pro-ductivity has declined by at least 6% to 12%, probably asa result of increased exposure to ultraviolet radiation.(The problem of stratospheric ozone depletion is dis-cussed in detail in Chapter 20.)

Another human-induced change that may be respon-sible for declines in certain Antarctic populations isglobal warming. As the water has warmed in recentdecades around Antarctica, less pack ice has formed dur-ing winter months. Large numbers of marine algae are

CASE·IN·POINT

74 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

Figure 4.11 Antarctic krill (Euphausia superba). These tiny,shrimplike animals live in large swarms and eat photosyntheticalgae in and around the pack ice. Whales, seals, penguins, andfish consume vast numbers of krill. Photographed in a tank onPalmer Station, Antarctica.

Unintended Changes in Food Webs

Sometimes the best human intentions can backfire. At the NationalElk Refuge near Jackson, Wyoming, wildlife biologists made a man-agement decision to feed the elk during the bitter cold winters,thereby preventing large population die-offs. As a result, the elkpopulation has flourished in and around Jackson. But winter stillcauses natural die-offs, and the much larger elk population trans-lates into many elk carcasses in the area. Ravens, black birds thatare much larger than crows, are attracted to the elk carcasses,which are their main source of food during cold winter months. Thenumber of ravens has increased dramatically because of their amplewinter food supply.

In the spring, when the elk migrate northward to their summerranges, the large raven population migrates with them. Ravens areomnivores and feed on anything they can find—berries, smalllizards and snakes, insects, and songbird eggs. Because there arefewer elk carcasses in spring and summer months, the ravens areeating the eggs and babies of songbirds in much larger numbersthan before. This nest predation has resulted in a heavy toll onsongbird populations. Thus, feeding the elk at the National ElkRefuge has led to a decrease in songbirds outside the refuge.

E N V I R O B R I E F

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found in and around the pack ice, providing a criticalsupply of food for the krill, which spawn in the area.Years with below-average pack ice cover means less algae,which mean less krill spawning. Scientists have demon-strated that low krill abundance coincides with unsuc-cessful breeding seasons in penguins and fur seals, all ofwhich struggle to find food during warmer winters. Sci-entists are concerned that global warming may continueto decrease the amount of pack ice, which will reverber-ate through the food web. (Global warming, includingthe effect on Adelie penguins, is discussed in Chapter 20.)

To complicate matters further, some commercial fish-ermen have started to harvest krill, which is used to makefishmeal for aquaculture industries (discussed in Chapter18). Because so many marine animals depend on krill forfood, scientists worry that the human harvest of krill mayendanger many animals higher on the food web.

Ecological Pyramids

An important feature of energy flow is that as a result ofthe second law of thermodynamics, most of the energygoing from one trophic level to the next in a food chainor food web dissipates into the environment. Ecologicalpyramids often graphically represent the relative energyvalues of each trophic level. There are three main typesof pyramids—a pyramid of numbers, a pyramid of bio-mass, and a pyramid of energy.

A pyramid of numbers shows the number of organ-isms at each trophic level in a given ecosystem, withgreater numbers illustrated by a larger area for that sec-tion of the pyramid (Figure 4.12). In most pyramids ofnumbers, the organisms at the base of the food chain arethe most abundant, and each successive trophic level isoccupied by fewer organisms. Thus, in African grasslandsthe number of herbivores, such as zebras and wildebeests,

is far greater than the number of carnivores, such as lions.Inverted pyramids of numbers, in which higher trophiclevels have more organisms than lower trophic levels, areoften observed among decomposers, parasites, tree-dwelling herbivorous insects, and similar organisms. Onetree, for example, can provide food for thousands of leaf-eating insects. Pyramids of numbers are of limited useful-ness because they do not indicate the biomass of theorganisms at each level, and they do not indicate theamount of energy transferred from one level to another.

A pyramid of biomass illustrates the total biomassat each successive trophic level. Biomass is a quantitativeestimate of the total mass, or amount, of living material;it indicates the amount of fixed energy at a particulartime. Biomass units of measure vary: Biomass may berepresented as total volume, as dry weight, or as liveweight. Typically, pyramids of biomass illustrate a pro-gressive reduction of biomass in succeeding trophic levels(Figure 4.13). On the assumption that there is, on theaverage, about a 90% reduction of biomass for eachtrophic level, 10,000 kg of grass should be able to support1,000 kg of grasshoppers, which in turn support 100 kg oftoads. (The 90% reduction in biomass is an approxima-tion; actual field numbers for biomass reduction in naturevary widely.) By this logic, the biomass of toad eaters suchas snakes could be, at the most, only about 10 kg. Fromthis brief exercise, you can see that although carnivoresmay eat no vegetation, a great deal of vegetation is stillrequired to support them.

A pyramid of energy illustrates the energy con-tent, often expressed as kilocalories per square meter

T H E F LO W O F E N E R G Y T H R O U G H E C O S Y S T E M S 7 5

Fishing Down the Food Web

According to a 1998 study of changes in marine food webs overtime, fish being caught today are significantly lower on the foodchain than those caught 10 years ago. Dozens of fish species,mostly top predators such as cod, have been fished to commercialextinction, and today’s commercial fishermen are catching smaller,bonier, less-valuable species. The drop in the average trophic levelof commercial species indicates a loss of biological diversity andoverall resource exhaustion caused by overfishing. Fishermen areliterally fishing down the food web, now relying on species that eatplankton or invertebrates. If overfishing continues, even thesespecies will no longer be commercially available. Many marine biol-ogists, including the study’s authors, stress that fishing down thefood web is unsustainable. They argue that one way to reverse thismovement down marine food webs—and to preserve diversemarine environments—is to create protected areas where no fishingis allowed.

E N V I R O B R I E F TROPHIC LEVELNUMBER OFINDIVIDUALS

Secondaryconsumer

(bird of prey)

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10

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Figure 4.12 Pyramid of numbers. This pyramid is for a hypo-thetical area of temperate grassland. Based on the number oforganisms found at each trophic level, a pyramid of numbers isnot as useful as other ecological pyramids. It provides no infor-mation about biomass or energy relationships between onetrophic level and the next. (Note that decomposers are notshown.)

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per year, of the biomass of each trophic level (Figure4.14). These pyramids, which always have large energybases and get progressively smaller through succeedingtrophic levels, show that most energy dissipates into theenvironment when going from one trophic level to the

next. Less energy reaches each successive trophic levelfrom the level beneath it because some of the energy atthe lower level is used by those organisms to performwork, and some of it is lost. (Remember, no biologicalprocess is ever 100% efficient.) Energy pyramidsexplain why there are so few trophic levels: Food webs areshort because of the dramatic reduction in energy content thatoccurs at each trophic level. (The eating habits of humansas they relate to food chains and trophic levels are dis-cussed in Chapter 18 in “You Can Make a Difference:Vegetarian Diets.”)

Productivity of Producers

The gross primary productivity (GPP) of an ecosystemis the rate at which energy is captured during photosyn-thesis.1 Thus, GPP is the total amount of photosyntheticenergy captured in a given period of time. Of course,plants must respire to provide energy for their own lifeprocesses, and cellular respiration by plants acts as a drainon photosynthesis. Energy that remains in plant tissuesafter cellular respiration has occurred is called net pri-mary productivity (NPP). That is, NPP is the amountof biomass found in excess of that broken down by aplant’s cellular respiration for normal daily activities forsurvival. NPP represents the rate at which this organicmatter is actually incorporated into plant tissues forgrowth.

Only the energy represented by NPP is available forconsumers, and of this energy only a portion is actuallyused by them. Both GPP and NPP can be expressed asenergy per unit area per unit time (kilocalories of energyfixed by photosynthesis per square meter per year) or interms of dry weight (grams of carbon incorporated intotissue per square meter per year).

Ecosystems differ strikingly in their productivities(Figure 4.15 and Table 4.1). On land, tropical rainforests have the highest productivity, probably becauseof their abundant rainfall, warm temperatures, andintense sunlight. As you might expect, tundra with itsharsh, cold winters and deserts with their lack of precip-itation are the least productive terrestrial ecosystems.

76 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

10

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Producers(grass)

Figure 4.13 Pyramid of biomass. This pyramid is for a hypo-thetical area of temperate grassland. Based on the biomass ateach trophic level, pyramids of biomass generally have a pyra-mid shape with a large base and progressively smaller areas foreach succeeding trophic level. (Note that decomposers are notshown.)

48

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Primary consumers(herbivores)

Producers

Figure 4.14 Pyramid of energy. This pyramid representsenergy flow, which is the functional basis of ecosystem struc-ture. It indicates how much energy is present at each trophiclevel and how much is transferred to the next trophic level.Note the substantial loss of usable energy from one trophiclevel to the next. (Note that decomposers are not shown. Also,the 36,380 kcal/m2/year for the producers is gross primaryproductivity, or GPP, which will be discussed shortly.)

1 Gross and net primary productivities are referred to as primarybecause plants occupy the first trophic level in food webs.

Net primaryproductivity

(plant growthper unit areaper unit time)

Gross primaryproductivity

(totalphotosynthesisper unit areaper unit time)

Plantrespiration

(per unit areaper unit time)

= –

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Wetlands, swamps and marshes that connect terrestrialand aquatic environments, are extremely productive.The most productive aquatic ecosystems are algal beds,coral reefs, and estuaries. The lack of available nutrientminerals in some regions of the open ocean makes themextremely unproductive, equivalent to aquatic deserts.

T H E F LO W O F E N E R G Y T H R O U G H E C O S Y S T E M S 7 7

More Is Sometimes Less

Conventional ecological theory held that the more productive thehabitat, the greater the biodiversity it supported. Now ecologistsare seeing a recurring pattern worldwide: Habitats become moreproductive and diverse as resources increase, but at some point,diversity actually declines with increasing productivity. For example,the resource-poor depths of the abyssal plain of the Atlantic Oceanhave higher species richness than the productive shallow watersnear the coasts; intermediate depths exhibit the greatest diversity.Scientists have little solid data to help explain the pattern, whichholds true with rodents in Israel, birds in South America, and largemammals in Africa. Mathematical ecosystem models, however, sug-gest that a patchy distribution of resources reduces competitionand allows the coexistence of a greater variety of organisms. (Recallfrom Chapter 2 that a model is a formal statement that describesthe behavior of a process, thereby helping us to understand howthe present situation developed from the past or to predict thefuture course of events.) The bad news for global biodiversity is thatwe are constantly enriching our environment with inputs from fossilfuels, fertilizer, and human and livestock sewage. This continualenrichment may make the Earth’s ecosystems increasingly produc-tive, a shift that could cost the world a substantial loss of biodiver-sity. (Factors that affect species richness, the number of specieswithin an ecosystem, are discussed in Chapter 5.)

E N V I R O B R I E F

Figure 4.15 A measure of Earth’s primary productivity. The data are from a satellite launchedas part of NASA’s Mission to Planet Earth in 1997. The satellite measured the amount of plantlife on land as well as the concentration of phytoplankton (algae) in the ocean. On land, themost productive areas, such as tropical rain forests, are dark green, whereas the least produc-tive ecosystems (deserts) are yellow and orange. In the ocean and other aquatic ecosystems, themost productive regions are red, followed by orange, yellow, green, and blue (the least produc-tive). Data are not available for the gray area.

Net Primary Productivities (NPP) for Selected

Ecosystems

Average NPPEcosystem (g dry matter/m2/year)

Swamp and marsh 3,000Tropical rain forest 2,200Temperate evergreen forest 1,300Temperate deciduous forest 1,200Savanna 900Boreal (northern) forest 800Woodland and shrubland 700Agricultural land 650Temperate grassland 600Lake and stream 400Arctic and alpine tundra 140Desert and semidesert scrub 90Extreme desert (rock, sand, ice) 3

Table 4.1

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(Earth’s major aquatic and terrestrial ecosystems arediscussed in Chapter 7.)

Human Impact on Net Primary Productivity Humansconsume far more of the Earth’s resources than any of theother millions of animal species. Peter Vitousek and col-leagues at Stanford University calculated in 1986 howmuch of the global NPP is appropriated for the humaneconomy and therefore not transferred to other organ-isms. When both direct and indirect human impacts areaccounted for, humans are conservatively estimated touse 32% of the annual NPP of land-based ecosystemsand 40% of the annual land-based NPP using the “mostreasonable” definition of human appropriation. Essen-tially, humans’ use of global productivity is competingwith other species’ needs for energy. Our use of so muchof the world’s productivity may contribute to the lossof many species, some potentially useful to humans,

through extinction. Clearly, at these levels of consump-tion of the Earth’s resources, human population growthbecomes a serious threat to the planet’s ability to supportits occupants.

Vitousek’s influential work was revisited in 2001 byStuart Rojstaczer and colleagues at Duke University.Rojstaczer used contemporary data sets, many of whichare satellite-based and are more accurate measures thanthe data Vitousek had available for his ground-breakingresearch. Rojstaczer’s mean value for his conservativeestimate of annual land-based NPP appropriation byhumans was 32%, like Vitousek’s, although Rojstaczerarrived at that number using different calculations. Itmust be emphasized that both Vitousek’s and Rojstaczer’snumbers are estimates, not actual values. The take-homemessage is simple, however: If we want our planet tooperate sustainably, we must share terrestrial photosyn-thesis products—that is, NPP—with other organisms.

78 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

SUMMARY WITH SELECTED KEY TERMS

I. The study of the relationships between organisms (the bioticenvironment) and the nonliving (abiotic) environment is calledecology. Ecologists study individual organisms, populations,communities/ecosystems, landscapes, and the biosphere.

A. A population is all the members of the same species thatlive together in a particular area at a particular time.

B. A community is all the populations of different species liv-ing in the same area at the same time.

C. An ecosystem is a community and its abiotic environment.In an ecosystem, all of the biological, physical, and chemicalcomponents of an area form a complicated interacting net-work of energy flow and materials cycling.

D. Landscape ecology considers the connections amongecosystems found in a particular region. Landscapes arelarger land areas than individual ecosystems.

E. The biosphere is the layer of Earth that contains all livingorganisms. The organisms of the biosphere depend on oneanother and on the other realms of the Earth’s physical envi-ronment: the atmosphere (gaseous envelope), lithosphere(soil and rock), and hydrosphere (water).

II. Energy is the capacity to do work. Energy can be trans-formed from one form to another.

A. Potential energy is stored energy; kinetic energy isenergy of motion.

B. Energy can be conveniently measured as heat energy; theunit of heat energy is the kilocalorie (kcal).

III. The study of energy and its transformations is known asthermodynamics.

A. The first law of thermodynamics states that energy can beconverted from one form to another, but it can neither becreated nor destroyed. The first law explains why organisms

cannot produce energy but must continuously capture itfrom the surroundings.

B. The second law of thermodynamics states that disorder(entropy) continually increases in the universe as usableenergy is converted to heat, a lower-quality, less-usableform.1. The second law explains why no process requiring

energy is ever 100% efficient.2. In every energy transaction, some energy is dissipated as

heat, which contributes to entropy.

IV. All life depends on a continuous input of energy.

A. Plants, algae, and some bacteria capture radiant energy dur-ing photosynthesis and incorporate some of it into carbo-hydrate molecules. The chemical formula that summarizesphotosynthesis is: 6CO2 + 12H2O + radiant energy →C6H12O6 + 6O2 + 6H2O.

B. All organisms obtain the energy in carbohydrate and othermolecules by cellular respiration, in which molecules suchas glucose are broken down with the release of energy. Thechemical formula that summarizes cellular respiration is:C6H12O6 + 6O2 + 6H2O → 6CO2 + 12H2O + energy.

C. Although the sun is the energy source for almost all ecosys-tems, hydrothermal vents are an exception. The base ofthe food web in hydrothermal vents consists of certain bac-teria that chemosynthesize, obtaining energy and makingcarbohydrate molecules from inorganic raw materials.

V. Energy flow through an ecosystem is linear, from the sun toproducer to consumer to decomposer.

A. Much of this energy is converted to less-usable heat as theenergy moves from one organism to another, as stipulated inthe second law of thermodynamics.

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B. Organisms in ecosystems assume the roles of producer, con-sumer, and decomposer.1. Producers, or autotrophs, are the photosynthetic

organisms that are potential food resources for otherorganisms. Producers include plants, algae, and somebacteria.

2. Consumers, which feed on other organisms, are almostexclusively animals. Primary consumers, or herbi-vores, feed on plants; secondary consumers, or carni-vores, feed on primary consumers; detritus feeders, ordetritivores, feed on detritus, dead organic material.

3. Microbial decomposers, or saprotrophs, feed on thecomponents of dead organisms and organic wastes,degrading them into simple inorganic materials that canthen be used by producers to manufacture more organicmaterial. Both consumers and decomposers are het-erotrophs.

C. Trophic levels may be expressed in food chains or, morerealistically, food webs, which show the many pathwaysthat energy can take among the producers (the first trophic

level), primary consumers (the second trophic level), sec-ondary consumers (the third trophic level), and so on withinan ecosystem.

D. Ecological pyramids express the progressive reduction innumbers of organisms (pyramid of numbers), biomass(pyramid of biomass), and energy (pyramid of energy)found in successively higher trophic levels.

E. Gross primary productivity (GPP) of an ecosystem is therate at which organic matter is produced by photosynthesis.Net primary productivity (NPP) expresses the rate atwhich some of this matter is incorporated into plant bodies.1. NPP is less than GPP because of the losses resulting

from cellular respiration by plants.2. Scientists have estimated how much of the global NPP is

appropriated for the human economy and therefore nottransferred to other organisms. When both direct andindirect human impacts are accounted for, humans areconservatively estimated to use 32% of the annual NPPof land-based ecosystems.

T H I N K I N G A B O U T T H E E N V I R O N M E N T 7 9

1. Draw a food web containing organisms found in a Chesa-peake Bay salt marsh.

2. What is the difference between a community and anecosystem? Between an ecosystem and a landscape?

3. How are the following forms of energy significant toorganisms in ecosystems: (a) radiant energy, (b) mechani-cal energy, (c) chemical energy, (d) heat?

4. Use the water stored behind a dam to explain the con-cepts of potential and kinetic energy.

5. When coal is burned in a power plant, only 3% of theenergy in the coal is converted into light in a regularlightbulb. What happens to the other 97% of the energy?Explain your answer using the laws of thermodynamics.

6. Distinguish between photosynthesis and cellular respira-tion. Which organisms perform each process?

7. Why do deep-sea organisms cluster around hydrothermalvents? What is their energy source?

8. Why is the concept of a food web generally preferredover that of a food chain?

9. Could a balanced ecosystem be constructed that con-tained only producers and consumers? Only consumersand decomposers? Only producers and decomposers?Explain the reasons for your answers.

10. How have humans affected the Antarctic food web?11. Suggest a food chain that might have an inverted pyramid

of numbers—that is, greater numbers of organisms athigher trophic levels than at lower trophic levels.

12. Is it possible to have an inverted pyramid of energy? Whyor why not?

13. Relate the pyramid of energy to the second law of ther-modynamics.

*14. The NPP for a particular river ecosystem is measured at8,833 kcal/m2/year. Respiration by the aquatic producersis estimated as 11,977 kcal/m2/year. Calculate the GPPfor this ecosystem.

*15. Graph the information given in Table 4.1 on page 77.Would it be more appropriate to construct a line graph ora bar graph? Why?

* Solutions to questions preceded by asterisks appear in Appendix VII.

THINKING ABOUT THE ENVIRONMENT

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Visit our Web site at http://www.wiley.com/college/raven/

(select Chapter 4 from the Table of Contents) for links to moreinformation about the environmental issues surroundingAntarctica. Consider the opposing views of Japanese whalersand environmentalists who oppose whaling, and debate theissues with your classmates. You will find tools to help youorganize your research, analyze the data, think critically aboutthe issues, and construct a well-considered argument. Take a

Stand activities can be done individually or as part of a team, asoral presentations, as written exercises, or as Web-based (e-mail) assignments.

Additional on-line materials relating to this chapter, includingStudent Quizzes, Activity Links, Useful Web Sites, FlashCards, and more, can also be found on our Web site.

80 Chapter 4 E C O S Y S T E M S A N D E N E R G Y

TAKE A STAND

SUGGESTED READING

Dybas, C.L. “Undertakers of the Deep.” Natural History(November 1999). The body of a dead whale supports adiverse community of organisms on the ocean floor formany years.

Field, C. “Human Appropriation of Natural Systems.” Envi-ronmental Review, Vol. 9, No. 3 (March 2002). This promi-nent scientist who specializes in global ecology talks abouthuman use of photosynthesis products (i.e., NPP) for food,fibers, and grazing by domesticated animals.

Hinrichs, R.A. Energy: Its Use and the Environment, 3rd ed.Philadelphia: Harcourt College Publishers, 2002. The focusof this introductory text is the physical principles behindenergy use and its effects on the environment.

Johnson, K. “The Vital Link.” Living Planet (summer 2001).The health of many marine species in the Southern Oceanaround Antarctica is based largely on the population size ofsmall, shrimplike krill.

Lutz, R.A., and R.M. Haymon. “Rebirth of a Deep-SeaVent.” National Geographic, Vol. 186, No. 5 (November

1994). Undersea lava flows recently buried some hydrother-mal vent ecosystems off the west coast of Mexico. Theauthor follows their quick rebirth as nearby vent speciesmigrated into the area.

Ross, J.F. “Hardly a Mouse or a Molecule Moves Here withoutBeing Noticed.” Smithsonian (July 1996). The SmithsonianEnvironmental Research Center studies how the Chesa-peake Bay functions as an ecological unit.

Schoen, D. “Primary Productivity: The Link to GlobalHealth.” BioScience, Vol. 47, No. 8 (September 1997). Exam-ines an important question that scientists are currentlyaddressing: How terrestrial primary productivity willrespond to climate warming.

Tyson, P. “Neptune’s Furnace.” Natural History (June 1999).Describes an expedition to raise black smoker chimneysfrom the ocean floor so their geology and biology can bestudied further.

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